The present invention relates to a method for producing a semiconductor device using a solder alloy free of lead, and, in particular, using a solder alloy of a tin (Sn)-antimony (Sb) system.
A solder alloy generally requires sufficient bonding performance and corrosion resistance. In power semiconductor devices for power converter application, a solder alloy is used to join the back surface of a semiconductor chip to a conductor pattern disposed on a principal surface (front surface) of an insulative substrate that is a ceramic substrate having conductor patterns on the surfaces thereof. Such a solder alloy needs high strength against thermal fatigue, because large thermal strain develops in the soldering area. The back surface of the semiconductor chip joins to the conductor pattern on the surface of the insulative substrate in a face-bonding way, and thermal expansion coefficients are different in a semiconductor chip and in a conductor pattern. In addition, the semiconductor chip generates heat in a conducting period. Therefore, the soldering portion suffers from large thermal strain.
In power semiconductor devices installed in a power converter for power conversion in electric vehicles, the conductor pattern disposed on the other principal surface (a back surface) of an insulative substrate is joined to a heat sink plate made of a metal. Since the soldering area is very wide, the solder alloy used for this joint must exhibit excellent wettability. Further, in the joining area between the heat sink plate and the conductor pattern on the back surface of the insulative substrate, large thermal strain develops caused by the difference in thermal expansion coefficients of the insulative substrate (a ceramic substrate) and the heat sink plate. Since the soldering area in the joint between the heat sink plate and the conductor pattern on the back surface of the insulative substrate is large, the generated strain in the soldering area is larger than the strain that develops in the joint between the semiconductor chip and the conductor pattern on the front surface of the insulative substrate as mentioned earlier.
Recently, a solder alloy that does not contain lead (Pb) is in demand in view of environmental considerations. One of such known solder alloys is a tin (Sn)—antimony (Sb) alloy. A known solder alloy (see Japanese Unexamined Patent Application Publication No. H11-58066, for example) contains tin (Sn) as a principal component, and antimony (Sb) not more than 3.0 wt %, silver (Ag) not more than 3.5 wt %, germanium (Ge) not more than 0.1 wt %, and further, copper not more than 1.0 wt % or nickel not more than 1.0 wt % or the both elements. Another known solder alloy (see Japanese Unexamined Patent Application Publication No. 2003-94194, for example) contains germanium (Ge) in the range of 0.01 to 10 wt %, antimony in the range of 5 to 30 wt %, and tin (Sn) in the range of 65 to 90 wt %.
A tin (Sn)—antimony (Sb) alloy, having a peritectic point at 8.5 wt % of antimony (Sb) and a temperature of 245° C., is generally used with a composition containing antimony (Sb) within 8 wt %. Melting of the tin (Sn)—antimony (Sb) alloy occurs at temperatures between 232° C., the melting point of tin (Sn), and 245° C., the peritectic point. The liquid-solid coexistence region is narrow, the heat resistance is favorable, and mechanically superior performances can be obtained by increasing the antimony (Sb) content. A large content of antimony (Sb), however, results in a problem of low wettability upon soldering the alloy. Oxidation of a solder component such as tin (Sn) involves another problem of deteriorated bonding performance.